Boron Substituted Na 3 V 2 (P 1 -x B x O 4 ) 3 Cathode Materials with Enhanced Performance for Sodium-Ion Batteries.

The development of excellent performance of Na-ion batteries remains great challenge owing to the poor stability and sluggish kinetics of cathode materials. Herein, B substituted Na3 V2 P3 -x B x O12 (0 ≤ x ≤ 1) as stable cathode materials for Na-ion battery is presented. A combined experimental and theoretical investigations on Na3 V2 P3 -x B x O12 (0 ≤ x ≤ 1) are undertaken to reveal the evolution of crystal and electronic structures and Na storage properties associated with various concentration of B. X-ray diffraction results indicate that the crystal structure of Na3 V2 P3 -x B x O12 (0 ≤ x ≤ 1/3) consisted of rhombohedral Na3 V2 (PO4 )3 with tiny shrinkage of crystal lattice. X-ray absorption spectra and the calculated crystal structures all suggest that the detailed local structural distortion of substituted materials originates from the slight reduction of V-O distances. Na3 V2 P3-1/6 B1/6 O12 significantly enhances the structural stability and electrochemical performance, giving remarkable enhanced capacity of 100 and 70 mAh g-1 when the C-rate increases to 5 C and 10 C. Spin-polarized density functional theory (DFT) calculation reveals that, as compared with the pristine Na3 V2 (PO4 )3 , the superior electrochemical performance of the substituted materials can be attributed to the emergence of new boundary states near the band gap, lower Na+ diffusion energy barriers, and higher structure stability.